Christian Zoffoli via Postfix-users:
> Using a load balancer like HAProxy for MySQL connections allows
> balancing only on servers that are synchronized. Direct use of multiple
> MySQL hosts in Postfix does not allow for any checks.
If you want to load balance N mysql servers behind 1 load balancer,
you can specify that server N times in the Postfix's "hosts" drective.
That will end the "mysterious delays".
The alternative, retry immediately on a single IP address, would
require configuration changes (it cannot be the default) and some
code to implement that.
> Given this, the
> example of MySQL, which could also be the balancing of multiple LMTP
> connections to IMAP servers,
If you present only one server IP address to the Postfix LMTP client,
then that client will not immediately retry that IP address after
error, because that would normally be rude behavior. Thus there
will be delays.
If you present multiple IP addresses to the Postfix LMTP client,
then it will try them in random order until delivery succeeds. There
will be no delays. I'm not sure if the client deduplicates IP
addresses; a quick code search suggests it does not.
As with mysql, making the Postfix LMTP client retry immediately on
a single IP address would require configuration changes (it cannot
be the default) and some code to implement that.
It gets messy if we want to do this with every network dependency.
> occur with the use of the balancer under some load conditions. I am
> looking for information to address the problem since Postfix does not
> natively support clustered milters.
Clustered or otherwise, the Postfix Milter client isn't going to
retry after a failure mid-session. There is no way to 'replay' the
Postfix-to-Milter conversation. The SMTP server does not control
what happens, it has to respond in real time to events from a remote
SMTP client and relay any responses from the Milter.
The SMTP and LMTP clients can handle mid-session failures because
they are in complete control.
Wietse
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